The present invention relates to an alkaline storage battery such as a nickel-hydrogen storage battery or a nickel-cadmium storage battery, and more particularly, to a rectangular alkaline storage battery including a rectangular metal casing can having a group of electrode plates sealed therein, wherein positive electrode plates formed from electrode plate cores coated with a positive active material and negative electrode plates formed from electrode plate cores coated with a negative active material are stacked alternately into layers with separators sandwiched therebetween.
In order to increase internal volumetric efficiency of equipment using storage batteries, a rectangular alkaline storage battery has been developed as a replacement for a cylindrical alkaline storage battery having a group of spiral electrodes into which positive and negative electrode plates are coiled spirally with separators sandwiched therebetween. In this type of rectangular alkaline storage battery, a group of electrode platesxe2x80x94into which positive and negative electrode plates are alternately stacked with separators sandwiched therebetweenxe2x80x94are inserted into a rectangular metal casing can. Positive leads projecting from the positive electrode plates are connected to a positive terminal, and negative leads projecting from the negative electrode plates are connected to a negative terminal. Subsequently, an electrolyte is poured into the metal casing can, and an opening section is sealed with a sealing member.
Demand has rapidly increased for a rectangular alkaline storage battery of this type to serve as a power source for portable equipment such as a notebook computer. In association with an increase in demand, there has arisen a desire for a rectangular alkaline storage battery having greater capacity and longer life. To this end, as described in, e.g., JP-A-10-312824, a rectangular alkaline storage battery of this type has been manufactured through the following processes. Namely, two negative electrode plates are formed, in the right-side and left-side portions of a common strip-shaped core, respectively. The center of the core (i.e., a joint) is bent into a U-shaped form. A positive electrode plate is interposed between the two negative electrode plates that have been bent into a U-shaped form, with a separator sandwiched between the positive electrode plate and each of the negative electrode plates, thereby constituting a electrode plate unit. Positive electrode plates are interposed between electrode plate units with separators sandwiched therebetween, thus constituting a group of electrode plates. The group of electrode plates are inserted into the rectangular casing can along with an electrolyte, thus manufacturing a rectangular alkaline storage battery.
In a rectangular alkaline storage battery described in JP-A-10-312824, active material is eliminated from the sides of cores of the electrode plates which are provided at the outermost positions of a group of electrode plates and brought into contact with the casing can, thus uncovering the cores of the electrode plates disposed at the outermost positions of the group of electrode plates. The group of electrode plates can be inserted into the rectangular casing can without involvement of exfoliation of active material, even though the group of electrode plates are not covered with a metal cover. Consequently, although volumetric energy density can be improved by only the amount corresponding to an omitted metal cover, exfoliation of active material from the electrode plates disposed at the outermost positions of a group of electrode plates can be prevented when the electrode plates are inserted into the casing can.
However, if an attempt is made to uncover the core of the electrode plate by means of removing active material from the sides of the electrode plates disposed at the outermost positions of the group of electrode plates, which sides are to be brought into contact with the casing can, the binding strength of an active material layer applied over the side of the electrode plate opposite to the exposed side thereof is lowered. For this reason, there arises a problem of an active material layer being exfoliated from the side of the electrode plate opposite to the core-exposed side thereof through repeated recharging and discharging operations. Punching metal formed by opening a plurality of pores in a metal electrode plate is usually used for a electrode plate core. However, the binding force which binds the active material layer applied over the punching metal directly to the punching metal is weak. Hence, active materials applied over the respective sides of the punching metal are bound together. If an active material layer applied over one side of punching metal is removed, the binding strength of the active material layer remaining on the other side of the punching metal becomes weaker, with the result that the active material layer located on that side falls from the electrode plate.
The present invention has been conceived to solve the problem set forth and is aimed at providing a rectangular alkaline storage battery which inhibits exfoliation of an active material so as to maintain the binding strength of an active material layer remaining on the sides of the electrode plates located at the outermost positions of a group of electrode plates, the sides being opposite the core-exposed sides thereof.
To this end, the present invention provides a rectangular alkaline storage battery constituted by means of hermetically sealing, in a rectangular metal casing can, a group of electrode plates in which positive electrode plates formed from electrode plate cores coated with positive active material and negative electrode plates formed from electrode plates cores coated with negative active material are alternately stacked with separators sandwiched therebetween, wherein each of the electrode plate cores has a plurality of pores; outer sides of electrode plate cores disposed at the outermost positions of the group of electrode plates are exposed; and the pore ratio of the electrode plate cores disposed at the outermost positions are 10 to 40%. Preferably, the pores formed in the electrode plate cores disposed at the outermost positions account for a percent area of the electrode plate cores (hereinafter such a percent is called a xe2x80x9cpore ratioxe2x80x9d) are lower than the pore ratio of respective electrode plate cores disposed inside of the outermost positions.
Here, in relation to the core of a electrode plate whose both sides are coated with active material, as pore ratio becomes greater, the permeability of gas evolved in a battery is improved. Moreover, the binding strength of active material provided on either side of each electrode plate core is also improved. Hence, one can safely say that pore ratio is to be increased to the extent that no drop arises in the strength of the electrode plate core.
However, in relation to the electrode plate core for which the applied active material layer is to be removed from one side thereof, as the pore ratio increases, gas permeability is improved. In contrast, the binding strength existing between active material and the electrode plate core drops, and active material falls from the electrode plates in association with discharging and recharging action. In relation to a electrode plate core in which the applied active material is to be removed from one side thereof, the lower the pore ratio, the greater the binding strength existing between the active material and the electrode plate core. In contrast, gas permeability drops.
For these reasons, the pore ratio of an exposed electrode plate core must be made lower than that of another unexposed electrode plate core. Also, the maximum and minimum pore ratios must be optimized. Various tests which have been performed show that a pore ratio of the exposed core of 10% or more inhibits deterioration of battery capacity, which would otherwise be caused in association with a decrease in electrolyte, without involvement of a drop in gas permeability. Further, it is also found that a pore ratio of the exposed electrode plate core of 40% or less improves gas permeability without involvement of a drop in the binding strength existing between active material and the electrode plate core.
When the pore ratio of a electrode plate core is less than 10%, the binding strength existing between the active material and the electrode plate core is increased. However, the permeability of the gas developing in the battery drops. In association with an increase in internal pressure during discharging and recharging cycles, the outer casing can expands, thus lowering the coverage of electrolyte to thereby deteriorate the battery capacity. In contrast, if the pore ratio of the electrode plate core exceeds 40%, the binding strength existing between the active material and the electrode plate core drops, with the result that the active material drops from the electrode plate in association with discharging and recharging operations.
In this case, bumps are formed along brims of respective pores formed in one side of each of the electrode plate cores located at the outermost positions of a group of electrode plates. Active material is applied over the side of the electrode plate having the bumps formed therein. If the side of the electrode plate opposite the bump-formed side is exposed, the bumps become buried in the active material layer, thereby enhancing the binding strength existing between the active material layer and the electrode plate core. For this reason, even when the active material layer is removed from one side of each of the electrode plate cores located at the outermost positions of the group of electrode plates, thus uncovering the electrode plate core, exfoliation of the active material layer from the side opposite the thus-exposed side can be inhibited to a much greater extent. If minute bumps are formed on the surfaces of the electrode plate cores located at the outermost positions of the group of electrode plates, excluding the pores formed therein, the minute bumps become buried in a layer of applied active material, thereby greatly enhancing the binding strength existing between the active material layers and the electrode plate cores located at the outermost positions.
When the exposed surface of each of the electrode plate cores located at the outermost positions of the group of electrode plates remains in contact with the interior surface of the rectangular metal casing can, the group of electrode plates can be readily inserted into the rectangular casing can without use of a metal cover and without involvement of exfoliation of active material. As a result, the volumetric energy density of the battery is improved by the amount corresponding to an omitted metal cover, and the efficiency of collecting electricity from the electrode plates located in the outermost positions to the metal casing can is improved. In this case, if the electrode plate core is constituted of punching metal, the electrode plate core of this type can be manufactured readily, because the punching metal can be formed very easily.
A nonporous joint section is formed integrally with each of the electrode plate cores located at the outermost positions of a group of electrode plates. Further, the joint section is formed into a substantially U-shaped form. A electrode plate of the other polarity is held in a substantially U-shaped space defined through bending, with separators sandwiched therebetween. Adoption of such a structure enables easy construction of a group of electrode plates of this type. Further, contact existing between the substantially U-shaped joint section and the interior surface of the bottom of the metal casing can be improved, thereby improving an efficiency of collecting electricity.